A long time ago, I started talking about "different webs," which (even in 2005) seemed to me to be a handy way to see the web in bigger terms than the somewhat squinty view of the web marketed as "web 2.0." Lately I've been describing different webs pretty simply, like:

The World Wide Web is the global platform for creating different webs. Each web "site" is its own, different, web; relationships between multiple sites create their own, different, webs; and relationships between sites and web services create their own, different, webs.

In Marc Andreessen's announcement, there is a faux Q & A which includes one good example of how and why web creators make different webs:

Are people really going to maintain multiple sets of front-end pages for their web sites for Facebook, Open Social, etc.?

I think so, yes. I think any web site going forward that wants maximum distribution across the largest number of users will have a single back-end, and then multiple sets of front-end pages:

One set of standard HTML and Javascript pages for consumption by normal web browser.

Another set of HTML and Javascript pages that use the Open Social API's Javascript calls for consumption with Open Social containers/social networks.

A third set of pages in FBML (Facebook Markup Language) that use Facebook's proprietary APIs for consumption within Facebook as a Facebook app.

Perhaps a fourth set of pages adapted for the Apple iPhone and/or other mobile devices.

This topic also ties in nicely with Brian Fling's The Demise of Mobile??? post and subsequent comments on the Blue Flavor blog. In the comments, Blue Flavor's Jeff Croft makes a similar observation about the single back-end / multiple front-end pattern described above (though Jeff talks in terms of layers—which is better: the back-end / front-end model is a way oversimplified way to think about this).